This invention relates generally to radio frequency circuits, and more particularly to fabrication of microwave analog circuits and digital control circuits on a common substrate.
As it is know in the art, monolithic microwave integrated circuits are used in a variety of applications. Such circuits, including amplifiers, switches, phase shifters, attenuators, and the like often require control signals to be fed to the circuits to switch amplifiers on and off, change the state of a switch, vary a phase shift imparted to a signal propagating through a phase shifter, or vary the amplitude of a signal fed through an attenuator, for example. One particular application for these analog circuits is arranged to provide an active transmit/receive (T/R) module for use in a phased array antenna system.
A phased array antenna includes a plurality of such T/R modules arranged in an array and each having the capability to impart to a signal a selected differential phase and/or amplitude characteristic. In a transmit mode, such modules are fed a common signal through a common feed network to provide a plurality of signal portions, each of which is acted upon by a corresponding T/R module to produce a plurality of transmit signals which are used to form directed and generally collimated beams of electromagnetic energy. The modules are also used to electronically steer such beams by varying the phase characteristics of the signals. There are many particular arrangements for T/R modules. A common configuration of a T/R module includes a reciprocal phase shifter and an optional attenuator disposed in a common path. A pair of switches and a pair of amplifiers, one amplifier being a high-power transmit amplifier, whereas the other amplifier being a low-noise receive amplifier are used to provide a pair of switchable amplication signal paths. A transmit signal is typically fed through the phase shifter to one of the switching circuits. The switching circuits are controlled to steer such phase shifted transmit signal through the transmit amplifier, through the other switch and onto a radiating element, during a transmit mode. In a receive mode, the switching circuits steer a received signal from the radiating element to the receive amplifier and through the phase shifter.
A common type of phase shifter employed in such a T/R module is a digitally controlled phase shifter, which can provide, in response to a control word, incremental differential phase shift to a signal propagating therethrough. A common configuration of a digitally controlled phase shifter includes a plurality of switchable phase shift elements which are controlled by feeding a parallel digital word to the gates of a corresponding plurality of transistors to switch such phase shift elements (typically lumped element network or lines of transmission line) into or out of the path of the propagating signal. Similarly, digital attenuators switches and amplifiers commonly used in T/R circuits are directly or indirectly controlled by digital control signals.
The general approach to control such analog circuits is to use separate digital circuits mounted in a T/R module package to interface the analog circuit with a beam steering controller, for example. Digital outputs from the digital circuits are connected to the analog circuit via wire bonds.
It is inefficient to use separate digital and analog microwave integrated circuits to control the transmit/receive module. Separate circuits only increase the size of the package and also increase the number of interconnections required between the digital circuits and the monolithic microwave integrated circuits to accommodate each parallel digital control word which is required to each of the circuits. It would be desirable therefore, to integrate both digital and analog functions on a common integrated circuit substrate. The benefits of such an arrangement would be several including reducing package size and reducing the cost associated with numerous bonding operations.
There are several problems, however, with having digital and analog circuits on a common substrate. In general, monolithic microwave integrated circuits particularly those fabricated with GaAs have relatively low yields due to various factors. For example, such circuits are required to operate at very high frequencies over microwave regions where minor variations in component values or in parasitics can introduce unwanted operational effects in the integrated circuits. Moreover, with Group III-V materials, such as gallium arsenide, processing technologies are not as well developed at this juncture as are other material technologies, such as silicon.
The principal operating characteristics of monolithic analog microwave integrated circuits include power generation capabilities and/or low-noise operation. On the other hand, digital monolithic integrated circuits are generally fabricated for dense packaging and very high speed operation. A standard logic family for digital monolithic integrated circuits formed by metal electrode semiconductor field effect transistors on gallium arsenide substrates is a logic family known as Buffered FET Logic (BFL). A typical high-performance depletion mode MESFET used in a BFL logic circuit is generally fabricated with one micron long gates and with doping levels about (3.times.10.sup.17).sup.3 centimeters. With this particular technology, circuits made or provided with minimum geometric FETs and with typical fan outs and line lengths will dissipate about 5 milliwatts per gate and have gate delays around 100 picoseconds (ps). The yield on these devices, particularly with the high-performance requirements are also generally relatively low compared to yields with silicon based digital devices.
Therefore, to combine the digital monolithic integrated circuits with analog monolithic microwave integrated circuits, would necessitate the use of the high-performance logic family mentioned above. This would add little to improve the performance of the control functions for the analog monolithic microwave integrated circuits, but would most likely reduce overall monolithic microwave integrated circuit device yield. Moreover, the BFL logic family consumes a lot of power due in part to high speed operation. It would be desirable to maintain the digital monolithic integrated circuit portions at lower temperatures to permit higher power dissipation by the analog portion of such circuits. Moreover, lower power dissipation and minimum spacing of the digital circuits would increase the yields of such circuit and the yield of analog monolithic microwave integrated circuits incorporating such digital circuits.
However, there are basic differences between the requirements for analog monolithic microwave integrated circuits and digital monolithic integrated circuits. Amongst these differences are that analog MESFETs are fabricated to be low-noise and/or high-power devices. Whereas for digital transistors, the noise performance as well as power capabilities of such digital transistors are of minor importance. With digital MESFETs threshold voltage is a significant factor, since it determines switching states for the devices and thus the logic noise immunity or noise margin for the logic family. For analog MESFETs, threshold voltage is not a significant concern and can vary somewhat from device to device. In MESFET transistors, threshold voltage is controlled by etching the source-drain channel region prior to depositing of the gate electrode. In a digital MESFET, a shallow etch is provided to more easily control the threshold voltage of the digital FET, whereas in an analog MESFET, a deep etch is used. Accordingly, these requirements are incompatible. Moreover, those analog monolithic microwave integrated circuits, operating at high power dissipation require thick metal interconnects to prevent electromigration. Thick interconnects are also useful to lower resistivity of the interconnects for low or high power circuits. The lower resistivity interconnect improves noise performance and efficiency of the analog MMICs. With the interconnection for digital circuits, it is preferrable to provide thin interconnects to conserve chip space thus permitting the devices to be densely packed. Accordingly, these as well as other incapabilities between the processing of analog and digital circuits have heretofore discouraged combination of such devices on a single substrate.